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Contributing to Tokio

🎈 Thanks for your help improving the project! We are so happy to have you!

There are opportunities to contribute to Tokio at any level. It doesn't matter if you are just getting started with Rust or are the most weathered expert, we can use your help.

No contribution is too small and all contributions are valued.

This guide will help you get started. Do not let this guide intimidate you. It should be considered a map to help you navigate the process.

The dev channel is available for any concerns not covered in this guide, please join us!

Conduct

The Tokio project adheres to the Rust Code of Conduct. This describes the minimum behavior expected from all contributors. Instances of violations of the Code of Conduct can be reported by contacting the project team at moderation@tokio.rs.

Contributing in Issues

For any issue, there are fundamentally three ways an individual can contribute:

  1. By opening the issue for discussion: For instance, if you believe that you have discovered a bug in Tokio, creating a new issue in the tokio-rs/tokio issue tracker is the way to report it.

  2. By helping to triage the issue: This can be done by providing supporting details (a test case that demonstrates a bug), providing suggestions on how to address the issue, or ensuring that the issue is tagged correctly.

  3. By helping to resolve the issue: Typically this is done either in the form of demonstrating that the issue reported is not a problem after all, or more often, by opening a Pull Request that changes some bit of something in Tokio in a concrete and reviewable manner.

Anybody can participate in any stage of contribution. We urge you to participate in the discussion around bugs and participate in reviewing PRs.

Asking for General Help

If you have reviewed existing documentation and still have questions or are having problems, you can open a discussion asking for help.

In exchange for receiving help, we ask that you contribute back a documentation PR that helps others avoid the problems that you encountered.

Submitting a Bug Report

When opening a new issue in the Tokio issue tracker, you will be presented with a basic template that should be filled in. If you believe that you have uncovered a bug, please fill out this form, following the template to the best of your ability. Do not worry if you cannot answer every detail, just fill in what you can.

The two most important pieces of information we need in order to properly evaluate the report is a description of the behavior you are seeing and a simple test case we can use to recreate the problem on our own. If we cannot recreate the issue, it becomes impossible for us to fix.

In order to rule out the possibility of bugs introduced by userland code, test cases should be limited, as much as possible, to using only Tokio APIs.

See How to create a Minimal, Complete, and Verifiable example.

Triaging a Bug Report

Once an issue has been opened, it is not uncommon for there to be discussion around it. Some contributors may have differing opinions about the issue, including whether the behavior being seen is a bug or a feature. This discussion is part of the process and should be kept focused, helpful, and professional.

Short, clipped responses—that provide neither additional context nor supporting detail—are not helpful or professional. To many, such responses are simply annoying and unfriendly.

Contributors are encouraged to help one another make forward progress as much as possible, empowering one another to solve issues collaboratively. If you choose to comment on an issue that you feel either is not a problem that needs to be fixed, or if you encounter information in an issue that you feel is incorrect, explain why you feel that way with additional supporting context, and be willing to be convinced that you may be wrong. By doing so, we can often reach the correct outcome much faster.

Resolving a Bug Report

In the majority of cases, issues are resolved by opening a Pull Request. The process for opening and reviewing a Pull Request is similar to that of opening and triaging issues, but carries with it a necessary review and approval workflow that ensures that the proposed changes meet the minimal quality and functional guidelines of the Tokio project.

Pull Requests

Pull Requests are the way concrete changes are made to the code, documentation, and dependencies in the Tokio repository.

Even tiny pull requests (e.g., one character pull request fixing a typo in API documentation) are greatly appreciated. Before making a large change, it is usually a good idea to first open an issue describing the change to solicit feedback and guidance. This will increase the likelihood of the PR getting merged.

Cargo Commands

Due to the extensive use of features in Tokio, you will often need to add extra arguments to many common cargo commands. This section lists some commonly needed commands.

Some commands just need the --all-features argument:

cargo build --all-features
cargo check --all-features
cargo test --all-features

Ideally, you should use the same version of clippy as the one used in CI (defined by env.rust_clippy in ci.yml), because newer versions might have new lints:

cargo +1.65.0 clippy --all --tests --all-features

When building documentation normally, the markers that list the features required for various parts of Tokio are missing. To build the documentation correctly, use this command:

RUSTDOCFLAGS="--cfg docsrs" RUSTFLAGS="--cfg docsrs" cargo +nightly doc --all-features

To build documentation including Tokio's unstable features, it is necessary to pass --cfg tokio_unstable to both RustDoc and rustc. To build the documentation for unstable features, use this command:

RUSTDOCFLAGS="--cfg docsrs --cfg tokio_unstable" RUSTFLAGS="--cfg docsrs --cfg tokio_unstable" cargo +nightly doc --all-features

The cargo fmt command does not work on the Tokio codebase. You can use the command below instead:

# Mac or Linux
rustfmt --check --edition 2021 $(git ls-files '*.rs')

# Powershell
Get-ChildItem . -Filter "*.rs" -Recurse | foreach { rustfmt --check --edition 2021 $_.FullName }

The --check argument prints the things that need to be fixed. If you remove it, rustfmt will update your files locally instead.

You can run loom tests with

cd tokio # tokio crate in workspace
LOOM_MAX_PREEMPTIONS=1 RUSTFLAGS="--cfg loom" \
    cargo test --lib --release --features full -- --test-threads=1 --nocapture

You can run miri tests with

MIRIFLAGS="-Zmiri-disable-isolation -Zmiri-tag-raw-pointers" \
    cargo +nightly miri test --features full --lib

Tests

If the change being proposed alters code (as opposed to only documentation for example), it is either adding new functionality to Tokio or it is fixing existing, broken functionality. In both of these cases, the pull request should include one or more tests to ensure that Tokio does not regress in the future. There are two ways to write tests: integration tests and documentation tests. (Tokio avoids unit tests as much as possible).

Tokio uses conditional compilation attributes throughout the codebase, to modify rustc's behavior. Code marked with such attributes can be enabled using RUSTFLAGS and RUSTDOCFLAGS environment variables. One of the most prevalent flags passed in these variables is the --cfg option. To run tests in a particular file, check first what options #![cfg] declaration defines for that file.

For instance, to run a test marked with the 'tokio_unstable' cfg option, you must pass this flag to the compiler when running the test.

$ RUSTFLAGS="--cfg tokio_unstable" cargo test -p tokio --all-features --test rt_metrics

Integration tests

Integration tests go in the same crate as the code they are testing. Each sub crate should have a dev-dependency on tokio itself. This makes all Tokio utilities available to use in tests, no matter the crate being tested.

The best strategy for writing a new integration test is to look at existing integration tests in the crate and follow the style.

Fuzz tests

Some of our crates include a set of fuzz tests, this will be marked by a directory fuzz. It is a good idea to run fuzz tests after each change. To get started with fuzz testing you'll need to install cargo-fuzz.

cargo install cargo-fuzz

To list the available fuzzing harnesses you can run;

$ cd tokio
$ cargo fuzz list
fuzz_linked_list

Running a fuzz test is as simple as;

cargo fuzz run fuzz_linked_list

NOTE: Keep in mind that by default when running a fuzz test the fuzz harness will run forever and will only exit if you ctrl-c or it finds a bug.

Documentation tests

Ideally, every API has at least one documentation test that demonstrates how to use the API. Documentation tests are run with cargo test --doc. This ensures that the example is correct and provides additional test coverage.

The trick to documentation tests is striking a balance between being succinct for a reader to understand and actually testing the API.

Same as with integration tests, when writing a documentation test, the full tokio crate is available. This is especially useful for getting access to the runtime to run the example.

The documentation tests will be visible from both the crate specific documentation and the tokio facade documentation via the re-export. The example should be written from the point of view of a user that is using the tokio crate. As such, the example should use the API via the facade and not by directly referencing the crate.

The type level example for tokio_timer::Timeout provides a good example of a documentation test:

/// // import the `timeout` function, usually this is done
/// // with `use tokio::prelude::*`
/// use tokio::prelude::FutureExt;
/// use futures::Stream;
/// use futures::sync::mpsc;
/// use std::time::Duration;
///
/// # fn main() {
/// let (tx, rx) = mpsc::unbounded();
/// # tx.unbounded_send(()).unwrap();
/// # drop(tx);
///
/// let process = rx.for_each(|item| {
///     // do something with `item`
/// # drop(item);
/// # Ok(())
/// });
///
/// # tokio::runtime::current_thread::block_on_all(
/// // Wrap the future with a `Timeout` set to expire in 10 milliseconds.
/// process.timeout(Duration::from_millis(10))
/// # ).unwrap();
/// # }

Given that this is a type level documentation test and the primary way users of tokio will create an instance of Timeout is by using FutureExt::timeout, this is how the documentation test is structured.

Lines that start with /// # are removed when the documentation is generated. They are only there to get the test to run. The block_on_all function is the easiest way to execute a future from a test.

If this were a documentation test for the Timeout::new function, then the example would explicitly use Timeout::new. For example:

/// use tokio::timer::Timeout;
/// use futures::Future;
/// use futures::sync::oneshot;
/// use std::time::Duration;
///
/// # fn main() {
/// let (tx, rx) = oneshot::channel();
/// # tx.send(()).unwrap();
///
/// # tokio::runtime::current_thread::block_on_all(
/// // Wrap the future with a `Timeout` set to expire in 10 milliseconds.
/// Timeout::new(rx, Duration::from_millis(10))
/// # ).unwrap();
/// # }

Commits

It is a recommended best practice to keep your changes as logically grouped as possible within individual commits. There is no limit to the number of commits any single Pull Request may have, and many contributors find it easier to review changes that are split across multiple commits.

That said, if you have a number of commits that are "checkpoints" and don't represent a single logical change, please squash those together.

Note that multiple commits often get squashed when they are landed (see the notes about commit squashing).

Commit message guidelines

A good commit message should describe what changed and why.

  1. The first line should:
  • contain a short description of the change (preferably 50 characters or less, and no more than 72 characters)
  • be entirely in lowercase with the exception of proper nouns, acronyms, and the words that refer to code, like function/variable names
  • be prefixed with the name of the sub crate being changed (without the tokio- prefix) and start with an imperative verb. If modifying tokio proper, omit the crate prefix.

Examples:

  • timer: introduce Timeout and deprecate Deadline
  • export Encoder, Decoder, Framed* from tokio_codec

  1. Keep the second line blank.

  2. Wrap all other lines at 72 columns (except for long URLs).

  3. If your patch fixes an open issue, you can add a reference to it at the end of the log. Use the Fixes: # prefix and the issue number. For other references use Refs: #. Refs may include multiple issues, separated by a comma.

    Examples:

    • Fixes: #1337
    • Refs: #1234

Sample complete commit message:

subcrate: explain the commit in one line

Body of commit message is a few lines of text, explaining things
in more detail, possibly giving some background about the issue
being fixed, etc.

The body of the commit message can be several paragraphs, and
please do proper word-wrap and keep columns shorter than about
72 characters or so. That way, `git log` will show things
nicely even when it is indented.

Fixes: #1337
Refs: #453, #154

Opening the Pull Request

From within GitHub, opening a new Pull Request will present you with a template that should be filled out. Please try to do your best at filling out the details, but feel free to skip parts if you're not sure what to put.

Discuss and update

You will probably get feedback or requests for changes to your Pull Request. This is a big part of the submission process so don't be discouraged! Some contributors may sign off on the Pull Request right away, others may have more detailed comments or feedback. This is a necessary part of the process in order to evaluate whether the changes are correct and necessary.

Any community member can review a PR and you might get conflicting feedback. Keep an eye out for comments from code owners to provide guidance on conflicting feedback.

Once the PR is open, do not rebase the commits. See Commit Squashing for more details.

Commit Squashing

In most cases, do not squash commits that you add to your Pull Request during the review process. When the commits in your Pull Request land, they may be squashed into one commit per logical change. Metadata will be added to the commit message (including links to the Pull Request, links to relevant issues, and the names of the reviewers). The commit history of your Pull Request, however, will stay intact on the Pull Request page.

Reviewing Pull Requests

Any Tokio community member is welcome to review any pull request.

All Tokio contributors who choose to review and provide feedback on Pull Requests have a responsibility to both the project and the individual making the contribution. Reviews and feedback must be helpful, insightful, and geared towards improving the contribution as opposed to simply blocking it. If there are reasons why you feel the PR should not land, explain what those are. Do not expect to be able to block a Pull Request from advancing simply because you say "No" without giving an explanation. Be open to having your mind changed. Be open to working with the contributor to make the Pull Request better.

Reviews that are dismissive or disrespectful of the contributor or any other reviewers are strictly counter to the Code of Conduct.

When reviewing a Pull Request, the primary goals are for the codebase to improve and for the person submitting the request to succeed. Even if a Pull Request does not land, the submitters should come away from the experience feeling like their effort was not wasted or unappreciated. Every Pull Request from a new contributor is an opportunity to grow the community.

Review a bit at a time.

Do not overwhelm new contributors.

It is tempting to micro-optimize and make everything about relative performance, perfect grammar, or exact style matches. Do not succumb to that temptation.

Focus first on the most significant aspects of the change:

  1. Does this change make sense for Tokio?
  2. Does this change make Tokio better, even if only incrementally?
  3. Are there clear bugs or larger scale issues that need attending to?
  4. Is the commit message readable and correct? If it contains a breaking change is it clear enough?

Note that only incremental improvement is needed to land a PR. This means that the PR does not need to be perfect, only better than the status quo. Follow up PRs may be opened to continue iterating.

When changes are necessary, request them, do not demand them, and do not assume that the submitter already knows how to add a test or run a benchmark.

Specific performance optimization techniques, coding styles and conventions change over time. The first impression you give to a new contributor never does.

Nits (requests for small changes that are not essential) are fine, but try to avoid stalling the Pull Request. Most nits can typically be fixed by the Tokio Collaborator landing the Pull Request but they can also be an opportunity for the contributor to learn a bit more about the project.

It is always good to clearly indicate nits when you comment: e.g. Nit: change foo() to bar(). But this is not blocking.

If your comments were addressed but were not folded automatically after new commits or if they proved to be mistaken, please, hide them with the appropriate reason to keep the conversation flow concise and relevant.

Be aware of the person behind the code

Be aware that how you communicate requests and reviews in your feedback can have a significant impact on the success of the Pull Request. Yes, we may land a particular change that makes Tokio better, but the individual might just not want to have anything to do with Tokio ever again. The goal is not just having good code.

Abandoned or Stalled Pull Requests

If a Pull Request appears to be abandoned or stalled, it is polite to first check with the contributor to see if they intend to continue the work before checking if they would mind if you took it over (especially if it just has nits left). When doing so, it is courteous to give the original contributor credit for the work they started (either by preserving their name and email address in the commit log, or by using an Author: meta-data tag in the commit.

Adapted from the Node.js contributing guide.

Keeping track of issues and PRs

The Tokio GitHub repository has a lot of issues and PRs to keep track of. This section explains the meaning of various labels, as well as our GitHub project. The section is primarily targeted at maintainers. Most contributors aren't able to set these labels.

Area

The area label describes the crates relevant to this issue or PR.

  • A-tokio This issue concerns the main Tokio crate.
  • A-tokio-util This issue concerns the tokio-util crate.
  • A-tokio-tls This issue concerns the tokio-tls crate. Only used for older issues, as the crate has been moved to another repository.
  • A-tokio-test The issue concerns the tokio-test crate.
  • A-tokio-macros This issue concerns the tokio-macros crate. Should only be used for the procedural macros, and not join! or select!.
  • A-ci This issue concerns our GitHub Actions setup.

Category

  • C-bug This is a bug-report. Bug-fix PRs use C-enhancement instead.
  • C-enhancement This is a PR that adds a new features.
  • C-maintenance This is an issue or PR about stuff such as documentation, GitHub Actions or code quality.
  • C-feature-request This is a feature request. Implementations of feature requests use C-enhancement instead.
  • C-feature-accepted If you submit a PR for this feature request, we wont close it with the reason "we don't want this". Issues with this label should also have the C-feature-request label.
  • C-musing Stuff like tracking issues or roadmaps. "musings about a better world"
  • C-proposal A proposal of some kind, and a request for comments.
  • C-question A user question. Large overlap with GitHub discussions.
  • C-request A non-feature request, e.g. "please add deprecation notices to -alpha.* versions of crates"

Calls for participation

  • E-help-wanted Stuff where we want help. Often seen together with C-bug or C-feature-accepted.
  • E-easy This is easy, ranging from quick documentation fixes to stuff you can do after reading the tutorial on our website.
  • E-medium This is not E-easy or E-hard.
  • E-hard This either involves very tricky code, is something we don't know how to solve, or is difficult for some other reason.
  • E-needs-mvce This bug is missing a minimal complete and verifiable example.

The "E-" prefix is the same as used in the Rust compiler repository. Some issues are missing a difficulty rating, but feel free to ask on our Discord server if you want to know how difficult an issue likely is.

Module

The module label provides a more fine grained categorization than Area.

  • M-blocking Things relevant to spawn_blocking, block_in_place.
  • M-codec The tokio_util::codec module.
  • M-compat The tokio_util::compat module.
  • M-coop Things relevant to coop.
  • M-fs The tokio::fs module.
  • M-io The tokio::io module.
  • M-macros Issues about any kind of macro.
  • M-net The tokio::net module.
  • M-process The tokio::process module.
  • M-runtime The tokio::runtime module.
  • M-signal The tokio::signal module.
  • M-sync The tokio::sync module.
  • M-task The tokio::task module.
  • M-time The tokio::time module.
  • M-tracing Tracing support in Tokio.

Topic

Some extra information.

  • T-docs This is about documentation.
  • T-performance This is about performance.
  • T-v0.1.x This is about old Tokio.

Any label not listed here is not in active use.

LTS guarantees

Tokio ≥1.0.0 comes with LTS guarantees:

  • A minimum of 5 years of maintenance.
  • A minimum of 3 years before a hypothetical 2.0 release.

The goal of these guarantees is to provide stability to the ecosystem.

Minimum Supported Rust Version (MSRV)

  • All Tokio ≥1.0.0 releases will support at least a 6-month old Rust compiler release.
  • The MSRV will only be increased on 1.x releases.

Versioning Policy

With Tokio ≥1.0.0:

  • Patch (1._.x) releases should only contain bug fixes or documentation changes. Besides this, these releases should not substantially change runtime behavior.
  • Minor (1.x) releases may contain new functionality, MSRV increases (see above), minor dependency updates, deprecations, and larger internal implementation changes.

This is as defined by Semantic Versioning 2.0.

Releasing

Since the Tokio project consists of a number of crates, many of which depend on each other, releasing new versions to crates.io can involve some complexities. When releasing a new version of a crate, follow these steps:

  1. Ensure that the release crate has no path dependencies. When the HEAD version of a Tokio crate requires unreleased changes in another Tokio crate, the crates.io dependency on the second crate will be replaced with a path dependency. Crates with path dependencies cannot be published, so before publishing the dependent crate, any path dependencies must also be published. This should be done through a form of depth-first tree traversal:

    1. Starting with the first path dependency in the crate to be released, inspect the Cargo.toml for the dependency. If the dependency has any path dependencies of its own, repeat this step with the first such dependency.
    2. Begin the release process for the path dependency.
    3. Once the path dependency has been published to crates.io, update the dependent crate to depend on the crates.io version.
    4. When all path dependencies have been published, the dependent crate may be published.

    To verify that a crate is ready to publish, run:

    bin/publish --dry-run <CRATE NAME> <CRATE VERSION>

  2. Update Cargo metadata. After releasing any path dependencies, update the version field in Cargo.toml to the new version, and the documentation field to the docs.rs URL of the new version.

  3. Update other documentation links. Update the "Documentation" link in the crate's README.md to point to the docs.rs URL of the new version.

  4. Update the changelog for the crate. Each crate in the Tokio repository has its own CHANGELOG.md in that crate's subdirectory. Any changes to that crate since the last release should be added to the changelog. Change descriptions may be taken from the Git history, but should be edited to ensure a consistent format, based on Keep A Changelog. Other entries in that crate's changelog may also be used for reference.

  5. Perform a final audit for breaking changes. Compare the HEAD version of crate with the Git tag for the most recent release version. If there are any breaking API changes, determine if those changes can be made without breaking existing APIs. If so, resolve those issues. Otherwise, if it is necessary to make a breaking release, update the version numbers to reflect this.

  6. Open a pull request with your changes. Once that pull request has been approved by a maintainer and the pull request has been merged, continue to the next step.

  7. Release the crate. Run the following command:

    bin/publish <NAME OF CRATE> <VERSION>

    Your editor and prompt you to edit a message for the tag. Copy the changelog entry for that release version into your editor and close the window.